It has been well established that insulation failures in oil filled equipment invariably increase pressure within the sealed enclosures housing the device (i.e. a capacitor or a transformer). However, external short-circuits, and even changes in the ambient temperature will also generate pressure variations, but tests with distribution transformers have shown that the pressure rise for external faults is gradual and pressure rises at a constant rate as long as fault current flows. For internal faults, a rapid pressure rise to dangerously high peak pressures can occur, for there is a transient surge in pressure inside a transformer when an internal arcing fault occurs.
Nearly all pole mounted distribution transformers are protected by fused cutouts that operate and visibly disconnect the transformer from the line if there is an overload or fault. When there is an internal fault, reenergizing the transformer can have serious consequences. In a worst case scenario, the tank can rupture, ejecting insulating oil and shards into the surroundings, thereby puffing the lineman at risk. There are special tools which must be used and procedures the lineman must follow when reenergizing a transformer to guard against undetected internal faults. In the past, the only foolproof method to check for internal faults was to disconnect the transformer and perform specific measurements.
Power utilities have adopted diverse inspection methods and other precautionary measures to reduce the hazards involved in reenergizing a faulted distribution transformer, but a much simpler and far safer solution is provided by recently developed internal fault indicators. For example, U.S. Pat. No. 5,078,078 to Nikola Cuk discloses a piston activated warning indicator which provides an indication that a transient overpressure condition has occurred within a transformer. Pressure within the transformer housing drives the piston outwardly from the housing and raises a cup-like member attached to the piston away from an indicator ring clip. It is intended that a lineman will observe this ring clip when an external inspection of the transformer occurs and thereby become aware of the fact that them has been a pressure surge due to an internal fault in the transformer.
Although the Cuk patented transformer internal fault indicator operates effectively in most instances, it is subject to several disadvantages. For a fault indication to be noticed, the cup-like member must physically move outwardly away from the transformer housing for a distance sufficient to fully expose the underlying indicator ring clip, and must remain in this position until an inspection is made. Since the piston which drives the cup-like member operates against the bias of a spring, this spring attempts to return the cup-like member to its original position over the indicator ring clip. Any external force applied to the cup-like member may aid the spring in causing movement of the piston back inwardly of the transformer housing, thereby partially or totally obscuring the indicator ring clip. Also, since the indicator ring clip must be observed in the limited space between the transformer housing and the extended cup-like member, it is often not readily noticed, particularly from a distance.